Builds the path of a route under MJAPI's base path (MJAPI_BASE_PATH).
subpath must start with /. Use it for every route a middleware registers or compares against.
With { absolute: true } it returns the public URL of the route (MJAPI_PUBLIC_URL + subpath).
The options type is written out here, not imported from apiRoot.ts: packages that read this
file's types from source must not compile config.ts.
What the server hands each middleware at startup.